NaNo does exactly what it was meant to do--get you to write something, anything, even if it sucks. My problem has always been a lack of focus. I can START several stories, but I'm terrible at outlining, and even if I do I tend to veer off course anyway. I've only done it once, and didn't make the word count because I veered off course and didn't know where to go.
I ended up not doing it this year because I am working a weird schedule, plus I got about 500 words in and realized I needed at least a semblance of a plan before I could get anywhere, and I didn't have one. The two clearest stories in my head are the one about making a movie and one about privateers that I've now started to write twice but haven't finished yet because I'm just not sure where it's going. It's times like these that I wish I was better at outlining and planning the stories in my head, but invariably I just get scenes to the stories, not outlines.